1. Field of the Invention
The present invention relates to sensors for measuring parameters, and more particularly, to a load control system having a load control device (such as, a dimmer switch or a temperature control device) and a wireless sensing device (such as, a daylight sensor or a temperature sensor).
2. Description of the Related Art
Some load control systems allow for the control of one or more electrical loads in response to a parameter measured by a sensing device. Daylight sensors (i.e., photosensors) are often used to measure the total light intensity in a space of a building that is illuminated by both artificial light from a lighting load (such as an incandescent lamp or a fluorescent lamp) and daylight (i.e., sunlight) shining through a window, such that the light intensity of the lighting load may be controlled to adjust the total light intensity in the space. For example, a lighting control device may decrease the light intensity of the lighting load as the total light intensity increases, and vice versa. Daylight sensors are typically mounted to a ceiling in the space at a distance from the window. In addition, some load control systems include temperature control devices that are coupled to a heating, ventilation, and air conditioning system (HVAC) system for controlling a present temperature in a space of the building to a setpoint temperature. The temperature control devices may include internal temperature sensing circuits for determining the present temperature in the space or may determine the present temperature in response to one or more remotely-located temperature sensors.
In prior art load control systems, it was typically necessary to electrically couple the sensors to the respective load control devices via electrical wires (for power and communication). However, it is typically not desirable to locate the daylight or temperature sensors next to the pre-existing electrical wires or to run additional electrical wires between the sensors and the respective load control devices, especially, in retro-fit installations. As a result, some load control systems include “wireless” daylight and temperature sensors that may be battery-powered and may transmit digital messages using a wireless medium, such as, for example, radio-frequency (RF) signals.
Since the transmission of RF signals typically consumes a large amount of power, the lifetime of the batteries of the sensors is a function of the number of times that the sensors transmit digital messages, and thus can be greatly shortened if the sensors transmit digital messages too often. Therefore, there is a need for battery-powered sensors that have an acceptable battery life and are able to communicate wirelessly with other control devices of the load control system.